(a) expressing in the cell or providing to the cell (i) a fusion protein comprising the test protein, X, and a microdomain tag fragment of a fluorescent protein, wherein the microdomain tag fragment corresponds to one or more contiguous beta-strands of the fluorescent protein, and (ii) a complementary assay fragment of the fluorescent protein functionalized to be directed and localized to the subcellular component of interest, wherein the assay fragment corresponds to the remaining contiguous beta-strands of the fluorescent protein, and wherein the microdomain tag and complementary assay fragments self-complement to reconstitute the fluorescent protein if present in the same subcellular component; and,

(b) detecting fluorescence in the cell, and thereby detecting the localization of the test protein, X, to the subcellular component of interest.